The present invention relates to novel copolymers of alpha-olefins. More specifically, it relates to novel copolymers of ethylene with other alpha-olefins which have a polymodal molecular weight distribution wherein individual modes comprising the polymer have narrow molecular weight distributions.
For convenience, certain terms that are repeated throughout the present specification are defined below:
(a) Inter-CD defines compositional variation, in terms of ethylene content, among polymer chains. It is expressed as the minimum deviation (analogous to a standard deviation) in terms of weight percent ethylene from the average ethylene composition for a given copolymer sample needed to include a given weight percent of the total copolymer sample which is obtained by excluding equal weight fractions from both ends of the distribution. The deviation need not be symmetrical. When expressed as a single number, for example, 15% Inter-CD, it shall mean the larger of the positive or negative deviations. For example, for a Gaussian compositional distribution, 95.5% of the polymer is within 20 wt % ethylene of the mean if the standard deviation is 10%. The Inter-CD for 95.5 wt % of the polymer is 20 wt % ethylene for such a sample.
(b) Intra-CD is the compositional variation, in terms of ethylene, within a copolymer chain. It is expressed as the minimum difference in weight (wt) % ethylene that exists between two portions of a single copolymer chain, each portion comprising at least 5 wt % of the chain.
(c) Molecular weight distribution (MWD) is a measure of the range of molecular weights within a given copolymer sample. It is characterized in terms of at least one of the ratios of weight average to number average molecular weight, M.sub.w /M.sub.n, and Z average to weight average molecular weight, M.sub.z /M.sub.w, where: ##EQU1## Ni is the number of molecules of weight Mi.
Ethylene-propylene copolymers, particularly elastomers, are important commercial products. Two basic types of ethylene-propylene copolymers are commercially available; ethylene propylene copolymers and ethylene propylene terpolymers. Ethylene-propylene copolymers (EPM) are saturated compounds requiring vulcanization with free radical generators such as organic peroxides. Ethylene-propylene terpolymers (EPDM) contain a small amount of non-conjugated diolefin, such as dicyclopentadiene; 1,4-hexadiene or ethylidene norbornene, which provides sufficient unsaturation to permit vulcanization with sulfur. Such ethylene-propylene polymers that include at least two monomers, i.e., EPM and EPDM, will be hereinafter collectively referred to as ethylene-propylene copolymers.
These copolymers have outstanding resistance to weathering, good heat aging properties and the ability to be compounded with large quantities of fillers and plasticizers resulting in low cost compounds which are particularly useful in automotive and industrial mechanical goods applications. Typical automotive uses are tire sidewalls, inner tubes, radiator and heater hose, vacuum tubing, weather stripping, sponge doorseals and Viscosity Index (V.I.) improvers for lubricating oil compositions. Typical mechanical goods uses are for appliance, industrial and garden hoses, both molded and extruded sponge parts, gaskets and seals and conveyor belt covers. These copolymers also find use in adhesives, appliance parts as in hoses and gaskets, wire and cable and plastics blending.
As can be seen from the above, based on their respective properties, EPM and EPDM find many, varied uses. It is known that the properties of such copolymers which make them useful in a particular application are, in turn, determined by their composition and structure. For example, the ultimate properties of an EPM and EPDM copolymer are determined by such factors as composition, compositional distribution, sequence distribution, molecular weight, and molecular weight distribution (MWD).
The efficiency of peroxide curing depends on composition. As the ethylene level increases, it can be shown that the "chemical" crosslinks per peroxide molecule increases. Ethylene content also influences the rheological and processing properties, because crystallinity, which acts as physical crosslinks, can be introduced. The crystallinity present at very high ethylene contents may hinder processability and may make the cured product too "hard" at temperatures below the crystalline melting point to be useful as a rubber.
Milling behavior of EPM or EPDM copolymers varies radically with MWD. Narrow MWD copolymers crumble on a mill, whereas broad MWD materials will band under conditions encountered in normal processing operations. At the shear rates encountered in processing equipment, broader MWD copolymer has a substantially lower viscosity than narrower MWD copolymer of the same weight average molecular weight or low strain rate viscosity. Thus, there exists a continuing need for discovering polymers with unique properties and compositions.
For elastomer applications the processability is often measured by the Mooney viscosity. The lower this quantity the easier the elastomer is to mix and fabricate. It is desirable to have low Mooney yet to maintain a high number average molecular weight, M.sub.n, so that the polymers form good rubber networks upon cross-linking. For EP and EPDM, narrowing the molecular weight distribution results in the production of polymer with higher number average molecular weight at a given Mooney than the broader distribution polymer. In certain cases, the poor milling, calendering or extrusion behavior that results from the narrow MWD must be ameliorated. Rather than perform a MWD broadening which includes low molecular weight components which reduce M.sub.n, it is possible to broaden the MWD without disproportionately lowering M.sub.n. This is done by superposing one or more narrow MWD modes, i.e., different Mooney components, each of which contains no appreciable amount of low molecular weight polymer. The result is a polymodal molecular weight distribution comprised of narrow MWD polymer fractions of different molecular weights.
The present invention is drawn to a novel copolymer of ethylene and at least one other alpha-olefin monomer which copolymer is composed of several such MWD components each of which is very narrow. It is well known that the breadth of the MWD can be characterized by the ratios of various molecular weight averages. For example, an indication of the narrow MWD of each component in accordance with the present invention is that the ratio of weight to number average molecular weight (M.sub.w /M.sub.n) is less than 2. Alternatively, a ratio of the Z-average molecular weight to the weight average molecular weight (M.sub.z /M.sub.w) of less than 1.8 typifies a narrow MWD in accordance with the present invention. It is known that the property advantages of copolymers in accordance with the present invention are related to these ratios. Small weight fractions of material can disproportionately influence these ratios while not significantly altering the property advantages which depend on them. For instance, the presence of a small weight fraction (e.g., 2%) of low molecular weight copolymer can depress M.sub.n, and thereby raise M.sub.w /M.sub.n above 2 while maintaining M.sub.z /M.sub.w less than 1.8. Therefore, the component polymers, in accordance with the present invention, are characterized by having at least one of two characteristics; M.sub.w /M.sub.n less than 2 and M.sub.z /M.sub.w less than 1.8. To obtain narrow MWD, the copolymers in accordance with the present invention are preferably made in a tubular reactor. It has been discovered that to produce such copolymers requires the use of numerous heretofore undisclosed method steps conducted within heretofore undisclosed preferred ranges. Accordingly, the present invention is also drawn to a method for making the novel copolymers of the present invention.